Even the experts have to check occasionally on the correct gauge of wire for
a given marine DC load. The simplest method we’ve found uses the charts
below.

Select either the 10% or 3% voltage drop chart, based on the type of load
you are running.

Next, find the current consumption of the load on the vertical axis of the
chart.

Find the length of the circuit on the horizontal axis of the chart, noting
that the length is the “round trip” distance from the panel or
battery to the load and back.

The color of the graph at the intersection denotes the gauge of wire to
use.

We've included copper wire specifications which comply with the AWG standards
at the bottom. Of particular interest is the equation:

Voltage Drop = Current x Length x Ohms per foot

This simple equation allows you to calculate the voltage drop for a circuit
of any length and any current flow, if you know the resistance of the wire.
Finally, note that the amp capacity of the wire curtails using very short lengths
of wire for large current flows, as show by the "flat tops" of the
10% chart areas.

Wires are not run in engine spaces: Maximum current is 15% less in engine
spaces, which are assumed to be 20¼C hotter than nonengine spaces (50¼C
vs. 30¼C).

Conductors are not bundled: If three conductors are bundled, reduce maximum
amperage by 30%. If 4-6 conductors are bundled, reduce maximum amperage by
40%. If 7-24 conductors are bundled, reduce amperage by 50%.